Cathode ray tube having post acceleration



Feb. 27, 1962 FRENKEL CATHODE RAY TUBE HAVING POST ACCELERATION FiledOct. 25, 1957 mm 3 S Q INVENTOR.

LOTHAR FRENKEL BY BUCK-HORN ,CHEATHAM & BLORE ATTORNEYS United StatesPatent 3,023,336 CATI-IODE RAY TUBE HAVING POST ACCELERATION LotharFrenkel, Portland, Oreg., assignor to Tektronix, Inc, Portland, Oreg., acorporation of Oregon Filed Oct. 25, 1957, Ser. No. 692,483 8 Claims.(Cl. 313-78) This invention relates to a cathode ray tube having postacceleration and more'particularly to such a cathode ray tube in which abrighter image of any desired size within wide limits may be producedwithout causing spherical aberration in the image and in which thedeflection sensitivity may be made very high.

In cathode ray tubes particularly suitable for oscilloscopes, it isdesirable to produce a sharply focused image on the screen of the tubeof substantial size and brightness. With all other factors constant, thebrightness of the image depends upon the velocity with which theelectrons of the electron beams strike the screen of the tube.Increasing the velocity of the electrons in the beam before deflectionof the beam in order to brighten-the image, increases the deflectionenergy required to produce an image of a given size to thus reduce thedeflection sensitivity of the tube. It is also possible to increase thevelocity of the electrons in the beam after deflection of the beam.Previously known structures for accomplishing this. have, however,either resulted in a marked reduction in deflection sensitivity of thetube or have produced excessive spherical aberration in the resultingimage on the screen of the tube.

In accordance with the present invention, the electrons in the beam areaccelerated after deflection of the beam but such acceleration isaccomplished by the combination of an electrostatic electronaccelerating and converging lens with a magnetic converging lens, whichcombination of lenses has sufficient converging action to cause thedeflected beam to cross the path of an undeflected beam and produce areversed image, such path being referred to in this application as thebeam axis of the tube. The electrostatic lens is thus of the type whichaccelerates the electrons in the beam and also causes convergencethereof so as to bend the deflected beam toward the beam axis.Electrostatic lenses of this type, in general, produce substantialspherical aberration in the image. This is particularly true when suchlens isemployed for electron acceleration after deflection of the beamso as to operate with a large efiective aperture. There is usually acritical point in the operating characteristic of such a lens at whichsubstantially no spherical aberration is produced. Operating at suchpoint, however, restricts the image on the screen of the tube to a fixedsize for a given value of beam deflection. Such a lens can also beoperated under conditions in which it produces spherical aberration ofthe pincushion type and the electrostatic lens of the present inventionis operated under such conditions.

Magnetic beam converging lenses, in general, produce sphericalaberration of the barrel type. By employing a magnetic converging lensposition to bend the deflected beam so as to also operate at a largeeflective aperture in combination with the electrostatic electronaccelerating and converging lens discussed above, the two sphericalaberration effects can be made to compensate for each other and thecombined action of the two lenses can be employed to cause the deflectedbeam to cross the beam axis and produce an inverted image withoutspherical aberration. By bringing the electrons in the beam to a focusin a focal plane prior to their entrance into the lens system, the imageformed at such focal plane can be projected in sharp detail upon thescreen of the tube by the lens system of the present invention. Becauseof the acceleration effect of the lens system, the electrons strike thescreen with greater velocity and the image is brighter for an image ofgiven size than would be the case without the electron accelerating andconverging lens combination of the present invention. Also, it ispossible to vary the size of the image on the screen to either produce asmaller image or a larger image than that which would be pro duced by agiven initial deflection in the absence of the electron accelerating andconverging lens combination of the present invention. It follows that abrightened image of the same size as that produced without such lens combination may be produced. Within limits a larger or magnified image canbe made brighter than the image produced without the lens combination ofthe present invention. Thus, by adjusting both the magnetic lens and theelectrostatic lens, images of any size within wide limits and ofincreased brightness may be produced on the screen of the cathode raytube for a given value of deflection of the beam and without sphericalaberration. With images of the larger size, the deflection sensitivitybecomes very large.

It is therefore an object of the invention to provide an I improvedcathode ray tube in which post acceleration of I ray tube in which acombination of an electrostatic electron accelerating and converginglens with a magnetic converging lens is employed toproduce either abrighter. image or magnified image, or both, without decreasing thedeflection sensitivity of the tube or distorting the image.

A further object of the invention is to provide a cathode ray tube foroscilloscopes in which the image produced by the deflection of a beam isbrought to a focus at a focal plane intermediate the length of the tubeand a post electron accelerating and converging lens system is employedto project such image upon the screen of a cathode ray tube whileproducing acceleration of the electrons in the deflected beam andenabling adjustment of the size of the image upon the screen and alsopreventing any substantial spherical aberration.

A still further object of the invention is to provide an improvedcathode raytube for oscilloscopes in which the image projected upon thescreen of such tube has its brightness increased without producingspherical aberration of the image and in which the size of such imagecan be adjusted independently of the initial deflection of the beam.

Other objects and advantages of the invention will appear in thefollowing description of preferred embodiments thereof shown in theattached drawings of which:

FIG. 1 is a partial diagrammatic vertical section through the cathoderay tube in accordance with the present invention, showingdiagrammatically a portion of the associated circuit;

'FIG. 2 is a diagrammatic view illustrating the type of sphericalaberration which may be produced by an electrostatic electronaccelerating and converging lens;

FIG. 3 is a view similar to FIG. 2illustrating the type of sphericalaberration produced by a magnetic electron converging lens;

FIG. 4 is a view similar to FIG. 2 illustrating the type of undistortedimage which can be produced on the screen of a cathode ray tube by acombination of the two efiects of FIGS. 2 and 3; and

FIG. 5 is a view similar to FIG. 1 showing a modified type of tube. 7

Referring more particularly to the drawings, FIG. 1 shows a cathode raytube including a glass envelope 10 having a neck portion 12 in which ispositioned an electron gun 13. The electron gun includes a cathode 1-4,a control grid 16, a pre-acceleration'grid 18, a first or focusing anode2t} and an accelerating anode 22. The electron gun illustrated is of thegeneral type known as a zero current first anode electron gun. Such agun is particularly suitable for employment with tubes of the presentinvention but any other known orsuitable type of electron gun can beemployed. The electron gun should be capable of producing a narrow beamof electrons which may be focused upon a focal plane indicated by adash-dot line 26 at a position intermediate the length of the glassenvelope 10. The beam 24 may be vertically deflected by a pair ofvertical deflection plates 28 positioned adjacent the electron gun 13and having deflection voltages applied thereto by suitable conductors(not shown). The beam may also be horizontally deflected by a pair ofhorizontal deflection plates 30 positioned between the verticaldeflection plates and the focal plane 26 and supplied with horizontaldeflection or sweep voltages through suitable conductors (not shown). Inthe absence of deflection of the beam, such beam will follow abeam axisindicated by the dotted line 32. The limits of vertical deflection ofthe beam are indicated by the dotted lines 24', such dotted linesdefining the vertical extent in FIG. 1 through which the deflected beam24 may travel rather than the boundary of a single beam of electrons. Itwill be apparent that similar limits of horizontal deflection can beshown in the horizontal plane so that an image is produced on the focalplane 26.

The deflected beam 24, after passing through the pairs of deflectionplates 28 and 30 and the focal plane 26, passes through an electrostaticaccelerating and converging lens 33 including a first open-endedcylindrical elemeht 34 and a second open-ended cylindrical element 36 ofgreater internal diameter and greater length than the element 34. Thetwo cylindrical elements are positioned so as to be concentric with eachother and the element 36 is positioned adjacent the element 34 butfurther along the beam than the element 34. The elements 34 and 36 areof conducting material and are connected to points of increasinglyhigher positive potential with respect to the accelerating anode 22 ofthe electron gun. Such elements constitute a known type of electronaccelerating and converging lens of the electrostatic type.

Positioned adjacent the element 34 and preferably exteriorly of the tubeso as to surround the envelope is a magnetic converging lens in the formof a coil 38 which is connected to a source of current through aresistor 40 engaged by an adjustable contact 41 to enable adjustment ofthe converging lens action of the coil 38. The coil 38 is preferablyconcentric with the elements 34 and 36 and its most effective positionalong the tube will usually be found tobe between the focal plane 26 andthe element 36 of the electrostatic lens 33.

All of the various elements in the envelope 10, including the elementsof the electron gun 13, the deflection plates 28 and 30 and the elements34 and 36 of the elec-' trostatic accelerating and converging lens 33are shown without their supporting elements for simplicity andclearness, and it will be understood that the required supportingelements may be of any known or suitable type and that the tube will besupplied with a suitable base. Also, the various elements of theelectron gun and electrostatic and accelerating converging lens areshown as being connected to a single resistor 42 with variable contacts44, 46, 47 and 48 thereon to provide for varying the bias on the controlgrid 16, the focusing voltage applied to the focusing anode and thevoltage applied to the elements 34 and 36 of the electrostatic lensassembly, respectively, but it will be understood that other types ofvoltage supplies may be employed. The accelerating anode 22 of theelectron gun may be connected to a conventional conducting coating 59 onthe interior surfaces of the glass envelope 10 and surrounding thedeflection plates 28 and 30. Also the end of the envelope remote fromthe electron gun may have a conventional fluorescent coating 52 formingafluorescent screen on the interior thereof.

In the tube above described, the electron gun produces a beam ofelectrons 24 which is directed toward the screen 52 of the tube throughthe deflection plates 28 and 30. The velocity of the electrons in thebeam during p ssage through the deflection plates may be of the order ofthat employed in conventional high sensitivity deflection cathode raytube for cathode ray Oscilloscopes, or may be even somewhat lower so asto provide for greater deflection sensitivity of the tube. Verticaldeflection voltages applied between the plates 28 in conjunction withhorizontal deflection or sweep voltages applied between the deflectionplates 30 cause the beam to be deflected vertically and horizontally,respectively, the boundaries of the vertical deflection of the beambeing indicated by the dotted line 24'. It will be understood that asimilar deflection is produced in the horizontal direction so that animage is produced. The voltage on the focusing anode 20 of the electrongun is adjusted so that such image is brought to a focus at the focalplane 26. The deflected beam continues forwardly through theelectrostatic lens elements 34 and 36. These elements are connected toprogressively higher points of positive voltage, such that the electronsin the deflected beams are accelerated and the deflected beam is benttoward the beam axis 32.

An electrbstatic accelerating and converging lens, such as that shown inFIG. 1, will, in general, produce spherical aberration of the imageprovided by the deflected beam of electrons. For one critical adjustmentof the voltages applied to the lens elements 34 and 36, a condition canusually be found in which no spherical aberration is present such thatan image produced by vertical and horizontal deflection voltages ofconstant amplitude related to each other so as to cover an area on thescreen would have an outline such-as shown in FIG. 4. For increasedvoltages on the elements 34 and 36 produced by varying the position ofthe contact 48" on the resistor element 42, the spherical aberrationtends to produce an image having an outline of the barrel type shown inFIG. 3 and such aberration increases with further decrease of suchvoltage. For decreased voltages on the elements 34 and 36, theaberration tends to produce an image having an outline of the pincushiontype shown in FIG. 2. For a magnetic converging lens, such as thatprovided by the coil 38, however, the aberration tends to produce animage having an outline of the barrel type shown in FIG. 3. Suchaberration becomes progressively greater as the current in the coil 38is increased to increase the converging effect. By employing both typesof lenses, the deflected beams can be made to cross the beam axis 32 ata crossover point 54 to' produce a reversed image upon the screen 52 ofthe cathode ray tube. By adjustment-of the positions of the contacts 47and 48 on the resistor ele ment 42 and the position of the contact 41 onthe resistor 40, positions of such controls can be found such that theimage on the screen 52 is of approximately the same size that it wouldbe if the magnetic and electrostatic lenses were omit-ted and such thatthe two types of spherical aberration discussed above compensate foreach other and the image is free from spherical aberration. Such imagewill be substantially brighter than the image produced in the absence ofthe lens system, since the electrons in the deflected beam have beenaccelerated, although the deflection sensitivity of the tube has notbeen changed. Thus a brighter image of given size may be produced for agiven deflection sensitivity.

By simultaneously adjusting the values of the positive voltages upon theelement 34 and 36 and at the same time adjusting the current in the coil38, a magnified image may be produced on the screen 52 which is freefrom spherical aberration and for a considerable degree of magnificationsuch image is still brighter than the image which would be produced inthe absence of the lens system. On the other hand, adjustment of thevoltage applied to the elements 34 and 36 and of the current in the coil33 can be made to produce a smaller image than that which would beproduced in the absence of the lens system and such image can be madefree from spherical aberration and substantially brighter than the imagepro duced in the absence of the lens system. It will, of course, beapparent that the movement of the contacts 41, 47 and 48 on theirrespective resistors may be coordinated so that they can be actuated bya single control. Thus, a brightened image of variable size may beproduced by the tube structure of the present invention without varyingthe deflection energy. required and without spherical aberration of theimage. 7

A modification of the lens system is shown in FIG. 5. In suchmodification, a glass envelope 56 is employed which has a cylindricalportion -7 spaced from but adjacent the screen 52. A conducting coating58 is applied to the interior of such cylindrical portion to form theequivalent to the cylindrical element 36 of the tube of FIG. 1. Also,the glass envelope 56 may have an inner concentric cylindrical glasselement 60 positioned rearwardly of the envelope 56 from the coating 58and such element 60 may have an interior conducting coating 62 formingthe equivalent of the lens element 34 of FIG. 1. The coil 38 forming themagnetic converging lens may surround-the envelope- 56 (FIG. 5) in thesame manner of the similar coil 38 surrounds the envelope of the tube ofFIG. 1. Thus, FIG. 5 forms a somewhat simpler structure which isequivalent to the structure of FIG. 1 and it will be apparent that theremainder of the structure of the tube of FIG. 5 may be the same, orsimilar, to that shown in FIG. 1. While a specific type of electron gunand a specific type of electrostatic accelerating and magneticconverging lens has been shown, it will be understood that othersuitable types of electron gun or electrostatic or magnetic converginglens known to those skilled in the art may be employed and that the lenssystem of the present invention may be employed with cathode ray tubeshaving magnetic focusing or magnetic deflection, or both.

While preferred embodiments of the invention have been disclosed, it isto be understood that the details thereof may be varied and that thescope of the invention is to be determined by the following claims.

I claim:

1. A cathode ray tube comprising an electron gun for producing a beam ofelectrons, deflection means for deflecting said beam to provide animage, electron accelerating means for accelerating the electrons insaid beam in a direction longitudinally of said beam after said beam hasbeen deflected by said deflection means, said electron acceleratingmeans including an electrostatic converging and accelerating lenssurrounding the deflected beam, said lens producing spherical aberrationof said image, and a magnetic converging lens also surrounding saiddeflected beam to correct for said aberration.

2. A cathode ray tube comprising an electron gun for producing a beam ofelectrons, deflection means for defleeting said beam to provide animage, electron accelerating means for accelerating the electrons insaid beam in a direction longitudinally of said beam after said beam hasbeen deflected by said deflection means, said electron acceleratingmeans including an electrostatic converging and accelerating lenssurrounding the deflected beam, said lens producing spherical aberrationof said image, and a magnetic converging lens also surrounding saiddeflected beam to correct said aberration, said lenses acting togetherhaving sufficient converging action to produce a reversal of said image.

3. A cathode ray tube comprising an electron gun for producing a beam ofelectrons, deflection means for deflecting said beam to provide animage, electron accelerating means for accelerating the electrons insaid beam in a direction longitudinally of said beam after asid beam hasbeen deflected by said deflection means, said electron acceleratingmeans including an electrostatic converging and accelerating lenssurrounding the deflected beam, said lens producing pincushion sphericalaberration of said image, and a magnetic converging lens alsosurrounding said deflected beam and producing compensating sphericalaberration to correct for the spherical aberration produced by saidelectrostatic lens, said lenses acting together having sufiicientconverging action to produce a reversal of said image.

4. A cathode ray tube comprising an electron gun for producing a beam ofelectrons in said tube directed along a beam axis, a screen in said tubeupon which said beam of electrons impinges, deflection means fordeflecting said beam out of said axis to produce an image, electronaccelerating lens means positioned between said deflection ,means andsaid screen for accelerating the electrons in the converging action toproduce a reversed and magnified image on said screen.

5. A cathods ray tube comprising an electron gun for producing a beam ofelectrons in said tube directed along a beam axis, a screen in said tubeupon which said beam of electrons impinges, deflection means fordeflecting said beam out of said axis to produce an image, electronaccelerating lens means positioned between said deflection means andsaid screen for accelerating the electrons in the deflected beam in adirection longitudinally of said beam, said electron accelerating lensmeans including an electrostatic converging and accelerating lensproducing spherical aberration of said image, and including a magneticconverging lens correcting for spherical aberration produced by saidelectrostatic lens, said electrostatic and magnetic lenses together tocause said electrons to strike said screen at greater velocity thantheir velocity during deflecting of said beam and having suflicientconverging action to produce a reversed image on said screen.

6. A cathode ray tube comprising an electron gun for producing a beam ofelectrons in said tube directed along I a beam axis, a screen in saidtube upon which said beam of electrons impinges, deflection means fordeflecting said beam out of said axis to produce an image, electronaccelerating lens means positioned between said deflection means andsaid screen for accelerating the electrons in the deflected beam in adirection longitudinally of said beam and bending said deflected beamtoward said axis, said electron accelerating lens means including anelectrostatic converging and accelerating lens producing pincushionspherical aberration of said image, and including a magnetic converginglens producing spherical aberration correcting for that produced by saidelectrostatic lens, said electrostatic and magnetic lenses actingtogether to cause said electrons to strike said screen at greatervelocity than the velocity of said electrons during deflection of saidbeam and having suflicient converging action to produce a reversed andmagnified image on said lens.

7. A cathode ray tube comprising an electron gun for producing a beam ofelectrons in said tube directed along a beam axis, deflection meansadjacent said gun for deflecting said beam out of said beam axis toproduce an image, focusing means for bringing said image to a focus in afocal plane intermediate the path of said beam in said tube, electronaccelerating and converging lens means surrounding the deflected beam onthe side of said focal plane opposite said deflection means, said lensmeans including an electrostatic accelerating and converging lensaccelerating the electrons in said deflected beam in a directionlongitudinally of said beam and bending said deflected beam toward saidaxis, said lens producing pincushion type spherical aberration of saidimage, said lens means also including a magnetic converging lens alsobending said deflected beam toward said axis and producing barrel typespherical aberration so as to correct the aberration produced by saidelectrostatic lens, a screen upon which the electrons of said beamimpinge after passing through said lens means, said lens means havingsufl'lcient converging action to cause the deflected beam to cross saidaxis and produce an inverted image on said screen.

8. A cathode ray tube comprising an electron gun for producing a beam ofelectrons in said tube directed along a beam axis, deflection meansadjacent said gun for deflecting said beam out of said beam axis toproduce an image, focusing means for bringing said image to a focus in afocal plane intermediate the path of said beam in said tube, electronaccelerating and converging lens means surrounding the deflected beam onthe side of said focal plane opposite said deflection means, said lensmeans including an electrostatic electron accelerating and beamconverging lens accelerating the electrons in said deflected beam in adirection longitudinally of said beam and bending said deflected beamtoward said axis, said lens producing pincushion type sphericalaberration of said image, said lens means also including a magneticconverging lens also bending said deflected beam toward said axis andproducing barrel type spherical aberration so as to correct foraberration produced by said electrostatic lens, a screen upon which theelectrons of said beam impinge after passing through said lens means,said lens means having suflicient converging action to cause thedeflected beam to cross said axis and produce an inverted image on saidscreen, and means for adjusting the converging effect of both of saidlenses to vary the size of said image on said screen without producingspherical aberration therein.

References Cited in the file of this patent UNITED STATES PATENTS FranceNov. 23,

